Cathodoluminescence is an excitation spectroscopy technique that consists in irradiating a luminescent specimen with an electron beam and measuring the induced emitted light. Within the interaction or generation volume of the incident electron beam with the specimen, the incident electrons undergo a series of elastic and inelastic scattering events, resulting in the generation of excited charge carriers i.e. electron-hole pairs. These electron-hole pairs diffuse and eventually recombine emitting photons; these photons form a cathodoluminescence signal.
The photons emitted by the recombining electron-hole pairs are detected using a photo-detector. By scanning the electron beam over the surface of the specimen and recording the cathodoluminescence signal intensity as a function of the electron beam position on the specimen, a spatially resolved cathodoluminescence map can be formed.
As the number of electron-hole pairs produced, and in turn the number of electron-hole pair recombination's which occur to emit photons, is dependent on the properties of the luminescent specimen, the cathodoluminescence map will thus be indicative of the properties of the luminescent specimen.
The cathodoluminescence signal can also be resolved spectrally. In a spectrally resolved cathodoluminescence experiment, a light-dispersing element disperses the cathodoluminescence signal and a single channel or a multi-channel photo-detector measures one specific spectral interval or multiple spectral intervals respectively. By scanning the electron beam over the surface of the specimen and recording the cathodoluminescence signal intensity over one on more spectral intervals as a function of the electron beam position on the specimen, a spatially and spectrally resolved cathodoluminescence map can be produced. Spectral information gives additional information about the luminescent properties of a specimen.
Cathodoluminescence is advantageous over purely optical excitation spectroscopy methods, such as photoluminescence, because it can feature higher spatial resolution; the highly focused electron beam of a scanning electron microscope can be used to excite a very small area of the specimen and thus information on the optical properties of a local area of the luminescent specimen can be obtained.
Yet, even with a nanometer size probe, the overall spatial resolution of a cathodoluminescence map is limited by the generation volume of the incident electron probe (the generation volume is the volume of the luminescent specimen which is excited by the incident electrons), and charge carrier diffusion within the specimen.
Indeed, depending on the material investigated, electron-hole pair recombination can occur microns away from the excitation spot, thus severely compromising the resolution of the cathodoluminescence map.
While it is extremely complicated to calculate charge carrier diffusion, the profile of the generation volume can be both accurately computed and measured. A Monte Carlo technique can simulate the electron trajectory within a specimen using probability distributions for scatterings events and charge carrier density thereof inferred. Luminescence theory can then relate the charge carrier density to the luminescence spectral intensity. FIG. 1 shows the result of two Monte Carlo simulations in bulk Gallium Nitride for (a) 1 keV and (b) 5 keV electron beam probe energy. It is known to embed quantum wells in a specimen to experimentally measure the generation volume.
A possible solution to improve the spatial resolution of cathodoluminescence maps is to work with low incident electron beam probe energy, e.g. a few keV or below. This reduces the generation volume of incident electrons. However, disadvantageously, at low acceleration voltages, only shallow subsurface features of the specimen can be accessed.
A solution to limit the impact of charge carrier diffusion on spatial resolution has been proposed. By operating the microscope in stroboscopic mode, and temporally gating the CL signal detection so that it only records the onset of the charge carrier diffusion process, they suggested that the spatial resolution could be improved. They used a beam blanker to pulse the electron beam. Unfortunately, such a technology cannot guarantee the stability of the beam while switched on and off; the space resolution starts to degrade for pulses having short temporal width (<1 ns). Since typical carrier mobility in semiconductors is of the order of nanometers per picosecond, the advantage of their technique is limited to materials having a large diffusion length (>1 μm).
D. S. H. Chan et al. disclose that confocal mirror optics might be used to collect the cathodoluminescence light (Review of Scientific Instruments 75 (2004), p. 3191). With such a solution, the resolution limit is no longer determined by the beam and specimen properties but by the light optics technology. Three-dimensional visualisation of the specimen is possible. Yet, the expected lateral resolution is of the order of a few hundred of nanometers and in-depth resolutions of the order of one micrometer at best. Thus, the solution does not provide satisfactory lateral and depth resolution.
Patent US2010059672 discloses how a 3D cathodoluminescence data set can be generated. US2010059672 discloses the use of an electron probe to excite the surface of a specimen and different measurement channels (e.g. EBSD, cathodoluminescence, secondary electrons etc.) to characterize it. An ion beam removes (by abrasion) the measured layer of the specimen. These operations are repeated as many times as required and a 3D cathodoluminescence can be reconstructed layer by layer. Disadvantageously, with such a method, the volume that is measured is destroyed; ions used for abrasion may penetrate the specimen and alter its optical properties; space resolution is limited by the size of the generation volume and by charge carrier diffusion.
US2004046120 discloses markers (nanoparticles) are injected in a cell so that they stick to different features of the cell. The markers are then observed with a cathodoluminescence microscope. A bright spot is indicative of the presence of a marker. A fuzzy appearance of the bright spots indicates that the markets are close to a membrane. This document teaches to deduce how far a marker is from a membrane using deconvolution.
Disadvantageously, the invention of US2004046120 does not yield spectroscopic information. The cathodoluminescence method is used to reveal the position of markers only, but does not give any information on the spectroscopic properties of the investigated specimen. US2004046120 therefore discloses generating 3D images whose contrasts depend on the structure of the specimen, but does not disclose how to generate 3D images whose contrasts depend on the spectral properties of the specimen. Furthermore, US2004046120 discloses measuring the cathodoluminescence of nanoparticles i.e. cathodoluminescence of nanoparticles showing up on cathodoluminescence spectra, but does not discloses measuring the cathodoluminescence of the specimen.
Pezzotti G. et al. in Micro/Nano Lithography look at the nanomechanical properties of electronic devices under the scanning electron microscope. This document discloses the extraction of stress information from a cathodoluminescence spectrum by applying known mathematical transformations. The convoluted extracted stress data is deconvoluted and a stress information is generated which is free from the blurring effect of the generation volume. The document is limited to disclosing how to deconvolute a stress map which shows mechanical information, to enhance its spatial resolution. The document fails to disclose how to deconvolute a spectrally resolved cathodoluminescence map (i.e. a set of cathodoluminescence data associated with different excitation positions) to enhance its spatial resolution. Furthermore, the document fails to disclose how to spatially deconvolve a spectrum. Additionally, the method disclosed is limited for use on a specimen with a very short diffusion length, as it is not disclosed how to reduce electron-hole diffusion artifacts.
It is an aim of the present invention to obviate or mitigate one or more of the aforementioned disadvantages.
The publication “Heterointerfaces in quantum wells and epitaxial growth processes; Evaluation by luminescence techniques” discloses how optical and structure properties of quantum-well heterostructures can be correlated in detail, and how these properties may be connected with the parameters of an epitaxial growth process. It is disclosed how luminescence techniques, mainly photoluminescence and cathodoluminescence imaging, may be used for evaluation of the structural disorder on the atomic scale, which occurs at the growth surfaces creating the interfaces of the quantum-well heterostructures.
The publication “A High Resolution Cathodoluminescence Microscopy Utilizing Magnetic Field” discloses a principle to improve spatial resolution of cathodoluminescence microscopy. The principle is to block the lateral diffusion by placing carriers in a circular orbit in terms of the Lorentz force under a vertical magnetic field.
The publication “Probing carrier dynamics in nanostructures by picoseconds cathodoluminescence” discloses the application of a time resolved cathodoluminescence set-up to describe carrier dynamics with a single gallium-arsenide-based pyramidal nanostructures with a time resolution of 10 picoseconds and a spatial resolution of 50 nanometres. The behaviour of the charge carriers are monitored.